[Brian]

Vise grips, as tight as you can applied to old stud[s]. Spray with your favourite penetrant. Using your judgement as to how much force to apply to visegips, apply torque back and forth to get stud moving and help penetrant seep down. If the stud[s] refuses to budge, gently apply heat to aluminum surrounding stud, and repeat the twisting motion. Work the stud back and forth a few times to loosen, don't simply crank it all the way out at once; it could seize and snap/strip the aluminum threads. Use your judgement.
To fit the new studs, use the double nut locking method. Could pay to chase the threads in manifold prior to fitting replacement studs.